This patent application relates to a method for manufacturing boron-free silicon dioxide by purification of hexafluorosilicic acid which is suitable as a base material for manufacturing silicon for semiconductor elements, particularly for solar cells.
The basic object of the invention is to specify a method for manufacturing boron-free (less than 1 ppm) silicon dioxide using inexpensive base materials such as fluoro-silicates which makes it possible to manufacture SiO.sub.2 for large-scale solar cell production.
The silicon obtained via large-scale production by means of reducing quartz (SiO.sub.2) using a carbon arc is not suitable for manufacturing solar cells from a purity point of view. The low purity of technical silicon (98% Si content) partially can be traced to the highly impure quartz used for its production.
From the German patent document DE-OS No. 30 13 319, a method for manufacturing silicon suitable for solar cells is well known. A quartz of high purity obtained by mining is used for reduction with use of carbon. Although the silicon manufactured this way has a substantially higher purity than the product obtained via large-scale production, an additional purification step, e.g. a crystal pulling process according to Czochralski, is also necessary in this case to remove the impurities which affect the element's parameters.
Boron-free SiO.sub.2 with a boron content of less than 1 ppm thus far could only be manufactured by means of gaseous silicon compound, such as silico-chloroform or via the glass phase; i.e. by melting the SiO.sub.2 with glass-forming compounds and extracting the vitreous body. Such a method has been described in German patent document DE-OS No. 29 45 141.
A purification of hexafluorosilicic acid via pyrolysis and subsequent hydrolysis of the obtained SiF.sub.4 in alkaline solution is described in German patent application No. P 32 06 766.6. With this method, the SiF.sub.4 is passed through a dioxane solution prior to hydrolysis to reduce the boron content whereby the boron-tri-fluoride contained in the SiF.sub.4 dissolves as ether.